The field of the invention is programmable controllers such as those disclosed in U.S. Pat. Nos. 3,942,158 and 3,810,118 and copending patent application Ser. No. 790,251 abandoned.
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored control program. In programmable controllers such as those disclosed in the above cited patents and patent application, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence to examine the condition of selected sensing devices on the controlled equipment and instructions which energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The processor in a programmable controller is designed to rapidly execute programmable controller type instructions which call for the manipulation of single-bit input data and the control of single-bit output data. Indeed, programmable controllers are distinguishable in this respect from process controllers which employ general purpose digital computers to control industrial equipment and processes. Whereas the primary function of a process controller may be to monitor a number of analog signals and control a number of analog operating devices, the primary function of programmable controllers is to monitor large numbers of single-bit digital devices, such as limit switches, and control a large number of single-bit operating devices such as solenoids and motor starters. As a result, the processor, or central logic unit of a programmable controller is typically less complex, less powerful and less costly than the processor of a general purpose computer, both because it processes single bits rather than words and because it is not called upon to perform substantial arithmetic functions. On the other hand, due to the large number of discrete sensing and operating devices connected to a programmable controller and the severe environment in which they are typically located, the I/O interface circuitry which couples industrial equipment to a programmable controller processor may be elaborate.
As programmable controllers grow in size and capability, they are finding increased application in industrial systems and processes which include analog devices and positioning devices which have heretofore been controlled by process controllers or special purpose hardwired control devices.
One such hardwired control device is the "programmable limit switch." Programmable limit switches are special apparatus which employ position transducers such as digital encoders or resolvers and which allow the presetting of certain angles at which a limit switch should be energized and the presetting of specific angles at which the limit switch should be denergized. They allow for multiple limit switches to be oriented along an axis and their trip position may be changed readily simply by programming or dialing in a different preset number. Such programmable limit switches are employed in place of cam operated limit switch assemblies. Although they are programmable, they are expensive devices which perform a very specific function. They do not perform other control functions on the machine and hence they must be used in combination with hardwired logic controllers or relay panels. The resulting control is expensive.